This invention relates to a centrifuge of the kind in which a liquid phase is withdrawn by means of a stationary skimmer disc, particularly but not exclusively a decanter having a separating space defined between an outer drum and an inner rotor which rotate at different angular velocities, the rotor carrying a screw conveyor which is operative in the separating space for discharging a solid phase therefrom.
When the liquid phase is withdrawn via a skimmer disc instead of being allowed to flow directly to the space surrounding the centrifuge drum it is, inter alia, possible to maintain a positive pressure in the discharge conduit which exceeds the pressure in the drum, and the formation of foam in the liquid discharged can be avoided or at any rate strongly limited. In the decanters mentioned above in which a solid phase is separated from a liquid phase present in the raw material and transported in the axial direction through the separating space by means of the screw until it is pushed out through an aperture at the end of the drum remote from the liquid outlet, it is furthermore possible to adjust the liquid level in the separating space by adjusting the positive pressure in the discharge conduit. The possibilities of adjustment are, however, rather limited and rather unreliable, inter alia because changes in the specific gravity of the liquid phase influence the location of the liquid level. Consequently, it has so far been the general practice to determine the liquid level by means of an annular disc which is secured to the drum and forms a weir at the liquid discharge end of the separating space. This purely geometric definition of the liquid level permits a very precise adjustment of the radial "height" of the liquid surface. It is, however, difficult to replace such a disc, particularly in the decanters mentioned above, in which at least the outer drum must necessarily be supported at both ends so that it is necessary to dismantle the main bearing of the drum together with a large number of additional components in order to gain access to the disc for replacing it with a disc of different central aperture.